Method of making precision wire-wound electrical resistors



Jan. 27, 1970 w. l. DETWEILER' ET AL 3,491,440

METHOD OF MAKING PRECISION WIRE-WOUND ELECTRICAL RESISTORS Original Filed June 12, 19 7 HHIIIIII INVENTORS Mu MM]. flzm z/zi/e BY '5/LL/E M Cam/441V g zm United States Patent O 3,491,440 METHOD OF MAKING PRECISION WIRE-WOUND ELECTRICAL RESISTORS William I. Detweiler, Thousand Oaks, and Billie M. Connally, Oxnard, Califi, assignors to Technology Instrument Corporation of California, Newbury Park, Calif., a corporation of California Original application June 12, 1967, Ser. No. 653,583, now Patent No. 3,427,548, dated Feb. 11, 1969. Divided and this applicationJune 25, 1968, Ser. No. 739,761 Int. Cl. Hlllc 17/00 US. Cl. 29-618 5 Claims ABSTRACT OF THE DISCLOSURE A resistance winding is accurately formed on an expanded split mandrel, and a split insulating shell surrounds and is bonded to the winding by a layer of thermosetting material. The bonding is accomplished by heating, and the mandrel is removed. The shell is fitted in a housing which has openings aligned with the slot in the shell, to permit terminals to be conductively connected to selected portions of the winding intermediate the edges of the slot.

This application is a division of our pending application, Resistor Winding Structure, Ser. No. 653,583, filed June 12, 1967, now Patent No. 3,427,548.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to improvements in potentiometers, variable resistors and the like, and more particularly to an improved method of making precision wire-wound electrical resistance windings.

Description of prior art In recent years, potentiome'ters and variable resistors have been developed which employ an internal design. In this design, a fine resistance wire is wound on a relatively large diameter supporting core of copper wire which is coiled in generally helical form. The movable wiper arm of the potentiometer or variable resistor is carried on a shaft within the helix and adapted to move longitudinally, upon relative rotation between the shaft and the helix, to contact the resistance wire at a selected point on a selected turn of the helix.

Compared with the conventional external design wherein the wiper arm is mounted for movement along the outside of the winding around a mandrel, the internal design permits potentiometers and variable resistors to be made which are much more compact, rugged and reliable, and lends itself to a greater degree of miniaturization and utility, where space and weight are at a premium.

Unfortunately, however, it is very difiicult, by present methods, to accurately wind and mount such resistance windings within a housing. Also, presently known methods are suificiently involved and unreliable that such structures cannot be made and sold except at considerable expense to both manufacturer and consumer.

In this latter connection, it has been attempted to form the helix as an integral part of a housing, as by potting a plastic material around the helix. When the potting material hardens, the result is a cylindrical housing with the helix embedded in its inner wall.

- In such a procedure, it is not possible to achieve quantity production of windings of uniform size and electrical characteristics. One reason is the difficulties inherent in trying to coil each helix so that its turns are precisely the same size and have the same pitch, and in supporting the helix so that its turns remain so during potting. Still another problem is that of damage to the fine ICC resistance wire which frequently occurs in removing hardened potting material, as where terminals and tap connections must be made to the winding at various points. To accomplish this requires that openings be made through the exterior of the housing to expose the wire at all points Where connections are to be made. Several procedures have been practiced to make such openings, including grinding and blasting techniques. As will be appreciated, meticulous care is required to make an opening that will expose the wire without damage thereto. However, it frequently happens that the resistance wire is damaged, and there is a high rejection rate of resistance windings.

SUMMARY OF THE INVENTION Our invention provides a simple, inexpensive, yet highly accurate method for manufacturing precision wound resistance windings for potentiometers and variable resistors employing the internal design, which can be repeated over and over again to uniformly produce resistance windings having the same structure and electrical properties. Our method provides a precision wound helical resistance winding with a slotted insulating shell bonded thereto, which facilitates electrically tapping the winding through a surrounding housing and the slot without damaging the resistance wire.

BRIEF DESCRIPTION OF THE DRAWING FIGURES 14 illustrate steps in the manufacture of a reuseable, slotted winding mandrel employed in our method of producing precision wound resistance windings having uniform structural and electrical characteristics, FIGURE 1 being a perspective view of a hollow sleeve prior to being heat treated, and, having a slot the length thereof, FIGURE 2 being a perspective view of the sleeve after heat treatment, wherein the sleeve is noncylindrical and the slot is closed, FIGURE 3 being a perspective view of the sleeve mounted on an arbor having an outer diameter the same as the original inner diameter of the sleeve to expand the sleeve to its original slotted shape, and FIG- URE 4 being a perspective view similar to FIGURE 3 with a helical groove in the outer surface of the sleeve and its ends partially relieved in line with the ends of the helical groove;

FIGURE 5 is a perspective view similar to FIGURE 4 illustrating the step of winding an electrical-resistance wire around the winding mandrel in the helical groove;

FIGURE 6 is a perspective view of a strip of cloth impregnated with thermosetting material to be placed around the winding mandrel over the resistance winding;

FIGURE 7 is a perspective view of a slotted shell for fitting over the outer surface of the strip shown in FIG- URE 6; 7

FIGURE 8 is a perspective view of the winding mandrel with the resistance wire wound thereon, the strip wrapped around the resistance winding, the shell over the strip, and the ends of the combination fixed between the collar around the arbor and a washer-nut'combination screwed tightly around the threaded end of the arbor;

FIGURE 9 is a perspective view of the winding mandrel, winding, strip and shell combination removed from the arbor with the mandrel in its unexpanded state;

FIGURE 10 is a perspective view of the winding, strip and shell removed from the winding mandrel; and

FIGURE 11 is a perspective view of the completed prevision wound resistance winding assembly made by our method, with the winding, strip, and shell within a cylindrical housing.

DESCRIPTION OF PREFERRED EMBODIMENTS To form the precision wound resistance winding of our invention, we utilize a novel manufacturing method em- 3 ploying a reuseable, slotted winding mandrel To produce the mandrel 10, a hole 12 is bored longitudinally through a sleeve 14 of hardenable tool steel, and a slot 16 the length thereof is cut in the sleeve. The slotted sleeve, which was a cylinder to begin with, is then compressed with a suitable tool so that the slot 16 is closed. While thus compressed, the sleeve is heat treated, preferably to 5556 Rockwell C; After being thus heat treated, the sieeve is of course a noncylindrical element.

To complete the formation of the winding mandrel 10, the sieeve 14 is forced onto an arbor 18 having an outer diameter the same as the original inner diameter of the sleeve 14. The sleeve is thus expanded to a cylinder, and the slot 14 takes its original shape. Next, a helical groove 20 is precision ground in the outer surface of the cylinder 14 as by turning the arbor 18 on a lathe while grinding with a cutting piece. Preferablyfthe end faces of the sleeve 14" are undercut, as at 22, where the groove 20 begins and ends. 7 e

The arbor 18 also forms the work-piece for making the winding structure of our invention. In this connection the arbor 18 is formed with a threaded end 19 of reduced diameter, and the split mandrel 10 is positioned on the; arbor so that one end thereof is flush with the radial transition face 21 between the threaded portion 19 and the main body of the arbor. T o aid in thus locating the mandrel, a collar or ring 23 is removably secured, as by a set screw, ontogthe arbor 18 a distance from the transition face 21 equal to the length of the split mandrel. Referring to FIGURE 5, one end of an elongated wirew-ound element 24 (e.g., a copper wire core on which fine resistance wire is helically wound in conventional fashion) is bent and located against the undercut portion of the mandrel 10 adjacent the ring 23. Then the element 24 is wrapped on the mandrel so that it fits in the groove 20, i.e., is formed into a helix, as by turning the arbor on a lathe while keeping the element 24 under tension.

When the winding reaches the other end of the mandrel 10, the end of the element 24 is clamped against that end of the mandrel by a washer 25 which is held in place by a nut 27 on the threaded end 10 of the mandrel 18.

Having secured the element 24 around the mandrel 10, a layer 26 of thermosefting material and a split shell 32 of insulating material, e.g., a plastic material such as glass epoxy, are placed with the ends of the layer and shell defining aligned slots. The layer 26 may be formed of a length of glass cloth which is impregnated with a thermosetting polyester resin. Preferably, the slot defined by the ends of the strip and shell are also aligned with the slot of the expanded mandrel. Also, the strip 26 may include holes, as at 30, through which to make external electrical, connections to the Wire-wound element 24 after the winding structure is completed. The shell also inciudes holes as at 36, which are aligned with the holes 30 in the strip 26.

Preparatory to heating, an element such as a rubber band may be placed around the shell 32 to aid in holding the parts in the assembled relation. The assembly is piaced in an oven and heated, e.g., at about 150 C. for about about forty-five minutes. The thermosetting material of the strip 26 softens and flows sufficiently to cover the surface of the wire-wound element '24 and the shell 32 which abut the strip 26, and results in continuous fillets bridging the element and the strip. Upon cooling, the theremosetting material cures to bond the shell, the strip and the element 24 into a unitary structure.

The mandrel 10 and the winding structure are removed from the arbor 18 by removing the collar 23, washer 25 and the nut 26. A suitable tool, such as a collar (not shown) having an outer diameter less than the outer diameter of the mandrel 10, is then slipped onto the arbor to engage one end of the mandrel. Opposing forces are exerted on the tool and the arbor 18, to force the mandre oflf the a bor.

i 4. e 7 When the mandrel 10 slides off the arbor 18, it auto matically constricts and returns to its unexpanded state as illustrated in FIGURE 9. It is then a simple matter to unscrew the unitary winding structure from the mandrel (see FIGURE 10). The mandrel 10 of course, is then ready for use in making another zprecision resistance winding structure having exactly the same structure and electrical characteristics as that previously formed.

The precision V wound resistance winding structure formed as above described may be utilized without change in a potentiometer or variable resistor, with terminals and taps not shown) seated in appropriate openings and welded to the resistance winding, and a wiper arm positioned within the winding'and mounted for longitudinal moveme'nt on a shaft which is rotatably mounted on the axis of the helix. If a sturdier structure is desired, the shell 32 may be ground to a smooth cylinder while mounted on the arbor 18, andthen, after removal from the arbor (and removal of the mandrel 10) press fit into a cylindrical 'inetal housing 44 having holes 46'aligned with the slot 34 and other holes 48 aligned with the holes 36 and 30 in the shell and strip. The terminals and taps extending through the holes are adapted for connection to an external circuit wiring, ail in a conventional manner.

In another embodiment of our invention, the layer 26 of thermosetting material is supplied by initially coating the interior surface of the shell 32 with such material. For coating the interior of the shell, we apply a thin coating of highly thixiotropic thermosetting adhesive, e.g., an epoxy resin approximately 0.004 thick. In this case, the ends of the layer define a slot that is inherently aligned with that defined by the ends of the shell 32, and inherently has openings aligned with the openings in the shell. The shell thus coated is placed around the winding as above described, and the assembly is heated to cause the thermosetting layer to soften and conform to the confronting portions of the winding. Such layer when heated sufiiciently wetsthe winding so that when cooled,'it securely bonds the shell and winding together.

From the foregoing, it will be seen that our invention provides a unique, simple and inexpensive method of making a precision resistance winding, and which may be reiiably repeated for volume production of such windings with the same electrical characteristics.

It will be apparent that modifications may be made in our method of manufacture without departing from the spirit and scope of our invention. We therefore do not intend that our invention shall be limited except in accordance with a reasonable interpretation of the appended claims.

We claim:

1. The method comprising the steps of:

forming a split metal cylindrical tube with .its ends spaced to define a slot;

compressing said tube to bring its ends into abutment;

heat treating the compressed tubeso it retains its radially compressed shape upon cooling;

expanding said heat treated tube to its original cylindrical shape;

forming a helical groove in the outer surface of said expanded tube,

wrapping a core with resistance winding thereon in said groove of said expanded tube;

placing a layer of thermosetting material and a surrounding, longitudinally split insulating sheli around the outer surface of said winding, the ends of said layer and shell defining aligned slots;

and heating said tube, layer and shell to allow said thermosetting material to flow and conform to said winding, said material, when set, bonding to said winding and the inner surface of said shell to form a unitary structure thereof.

2. The method of claim 1, wherein said layer is i111 tially coated on the inner surface of said shell, said coat-t ing being a highly thixiotropic thcrmosetting adhesive.

3. The method of claim 1, wherein said layer is a strip a of material impregnated with thermosetting resin.

4. The method of claim 1, wherein said expanding step includes forcing said tube onto a cylindrical arbor having an outer diameter equal to the inner diameter of the original metal cylinder, and including the further steps of:

forcing said expanded tube with the Winding and shell thereon off said arbor whereby the ends of said tube come together,

and removing said tube from within said winding.

5. The method of claim 3, further including the step of placing said shell in a cylindrical housing with openings therein aligned with the aligned slots in said shelland layer.

References Cited UNITED STATES PATENTS 2,558,326 6/1951 Van Dyke 338-449 2,645,701 7/1953 Kerr et a1 29-618 X 2,961,626 11/1960 Moore 338-1 43 3,028,571 4/1962 Mucher 338143 10 JOHN F. CAMPBELL, Primary Examiner I. L. CLINE, Assistant Examiner 

